Cytokines are proteins which are produced during the effector phases of natural and specific immunity and serve to mediate and regulate immune and inflammatory responses. Cytokines, like other polypeptide hormones, initiate their action by binding to specific receptors on the surface of target cells. One of the most well known families of cytokines are the interleukins which mediate natural immunity. For a detailed description of the structure and function of the interleukins, see Abbas et al. Cellular and Molecular Immunology, W. B. Saunders Company, Philadelphia, pp. 225–243, 1991.
Leukemia inhibitory factor (LIF), named for its ability to inhibit proliferation of a myeloid leukemic cell line by inducing differentiation, is a member of a family of ligands that includes IL-6, oncostatin M, ciliary neurotrophic factor (CNTF) and cardiotrophin-1 (Gearing, Adv. Immunol. 53:31–58, 1993; Pennica et al., J. Biol Chem. 270:10915–10922, 1995; Patterson, Proc. Natl. Acad. Sci. U.S.A. 91:7833–7835, 1994). Although these cytokines share only very limited sequence homology, they exert very similar effects on a variety of tissues. For example, several of these proteins, including LIF, can induce the same set of acute-phase response proteins in liver, support the self renewal of cultured embryonic stem cells, inhibit lipogenesis and enhance the survival of cultured motor neurons. LIF is produced by diverse cell populations, including macrophages, synoviocytes and chondrocytes. When applied to peripheral nerves in vivo, LIF is retrogradely transported and rescues damaged sensory neurons (Hendry et al., J. Neurosci. 12:3427–3434, 1992; Cheema et al., J. Neurosci. Res. 37:213–218, 1994). LIF also regulates the growth and differentiation of osteablasts and endothelial cells. The rising follicular fluid LIF level around the time of ovulation indicates that LIF may play a role in ovulatory events, early embryonic development and implantation (Senturk et al., Am. J. Reprod. Immunol. 39:144–151, 1998; Stewart, Annals N.Y. Acad. Sci. 157–165).
Neurotrophins and neurotrophic factors are proteins or peptides capable of affecting the survival, target innervation and/or function of neuronal cell populations (Barde, Neuron, 2:1525–1534, 1989). The efficacy of neurotrophins both in vivo and in vitro has been well documented. For example, ciliary neurotrophic factor (CNTF) promotes survival of chicken embryo ciliary ganglia in vitro and supports survival of cultured sympathetic, sensory and spinal motor neurons (Ip et al., J. Physiol. Paris, 85:123–130, 1991).
A major obstacle to the in vivo therapeutic use of peptides is their susceptibility to proteolytic degradation. Retro-inverso peptides are isomers of linear peptides in which the direction of the sequence is reversed (retro) and the chirality, D or L, of each amino acid is inverted (inverso). There are also partially modified retro-inverso isomers of linear peptides in which only some of the peptide bonds are reversed and the chirality of the amino acid residues in the reversed portion is inverted. The major advantage of such peptides is their enhanced activity in vivo due to improved resistance to proteolytic degradation (For review, see Chorev et al., Trends Biotech., 13:438–445, 1995). Although such retro-inverso analogs exhibit increased metabolic stability, their biological activity is often greatly compromised (Guichard et al., Proc. Natl. Acad. Sci U.S.A., 91:9765–9769, 1994). For example, Richman et al. (J. Peptide Protein Res., 25:648–662) determined that analogs of linear and cyclic leu-enkephalin modified at the Gly3—Phe4 amide bond had activities ranging from 6%–14% of native leu-enkephalin. Chorev et al., (ibid.) showed that retro-inversion of a peptide which inhibits binding of vitronectin to its receptor resulted in one peptide which was less potent than the parent isomer by a factor of 50,000, and another peptide which was 4,000 fold more potent than the parent cyclic peptide. Guichard et al. (TIBTECH 14, 1996), teach that retro-inverso (all-D-retro) antigenic mimicry may only occur with peptides in random coil, loop or cyclic conformations. In the case of “helical” peptide, adequate functional mimicry would be expected only if the helicity was, in fact, absent under the solvent conditions used for assessing antigenic mimicry.
There is a need for LIF-derived and neurotrophic peptides exhibiting increased metabolic stability while retaining biological activity. The present invention addresses this need.